This invention relates to a method of applying a porous metal coating to a metal substrate, and to a substrate, such as a heart valve or other prosthesis, having such a coating.
The anchoring of body tissue to certain synthetic implants in the body is desirable. Metals are used extensively in a variety of implant forms, such as artificial heart valves, hip joints, etc., making it necessary to anchor the body tissue to a metal structure.
The incidence of thrombo embolism poses a continual threat to patients who have undergone surgery for prosthetic heart valve replacement. Clinical evidence has shown that the incidence of thrombo embolism can be significantly decreased within one year after surgery if the prosthetic valve can be constructed to encourage a controlled tissue ingrowth over the surface of the valve exposed to the blood stream. This requires a porous surface.
The most commonly used technique for encouraging tissue growth in the case of a heart valve is to cover the surfaces of the valve with a knitted or woven cloth. However, cloth produced by current methods is limited in its durability and minimum thickness which can be achieved.
Flame spraying methods have also been attempted on metal heart valves and hip joints. In these methods a layer of particles is welded to a metal substrate by a spray of fused, minute droplets of metal. This procedure does not produce a desired uniformity of either porosity or thickness of coating. It generally produces very little porosity -- only a rough surface. Also, objectionable oxide inclusions normally result. It is also difficult at best with these methods to coat small selected areas with good definition. A coating having a thickness equivalent to one layer of beads, which is often desired, is very difficult if not impossible to produce by flame spraying methods.
Sintering has also been attempted wherein beads of metal are applied in a slurry with binder. This, again, results in poor uniformity of coating thickness and selected area coverage is difficult.
Objects of the present invention are, therefore, to provide a porous metal coating of uniform and controlled thickness and porosity on a metal substrate, to provide a coating of the type described consisting of one or more layers of metal particles in which the original shape and size of the particles are retained, to provide such a coating free of objectionable oxides or binders, to provide such a coating with clear definition on small selected areas of a substrate, to provide such a coating for tissue ingrowth in metal prosthetic devices, and to provide an improved method for applying such a coating.